Butterfly valves used for automatic control reduce pressure particularly at low flow, i.e., low disc openings. Such pressure reduction can only be accomplished by acceleration and subsequent deceleration of the passing medium, where the maximum velocity is a square root function of the pressure drop. Unfortunately, such high velocities along the leading edges of butterfly valves produce undesirable side effects. Such side effects are cavitation with liquid media, aerodynamic throttling noise with gases, and a high dynamic torque with either medium. Such dynamic torque, increasing rapidly towards the fully open position and reaching a peak near 70.degree. disc opening (see curve A in FIG. 4), greatly interferes with the stable valve operation particularly when pneumatic actuators are employed. This torque is a function of the suction effect (much like the "lift" of an airplane wing) produced by high velocity on the upper surface of that portion of the disc pointing in the downstream direction.
Past butterfly discs of improved designs have tried to overcome this dynamic torque problem, notably among them a disc whose terminating downstream periphery has the shape of a fishtail. Another design employs semi-circular cavities on opposing sides of a circular disc. While those designs show improvements in reduction of dynamic torque, neither of them meets the additional objectives of my invention, which are:
a. reduce hydrodynamic torque; PA1 b. provide tight shut-off; PA1 c. create a better control characteristic.
My invention produces substantially less dynamic torque than either of the present state of the art butterfly discs by providing two tiltingly offset semi-circular wall portions forming the basic disc.
With the vane sitting at a typical seat angle of between 15.degree.-20.degree. from an axis perpendicular to the cylindrical butterfly valve bore, I have the upper semi-circular disc portion approximately perpendicular to said bore and the lower semi-circular wall portion tilted at an approximate angle of 30.degree. in respect to the upper half.
As can be seen from FIG. 4 of my attached drawing, dynamic torque peaks at around 70.degree. of the angular displacement from the vertical axis for a flat vane. Above this angle, the torque is drastically reduced and reaches a negative value beyond 90.degree.. Two angularly offset vane halves accomplish that the upper, perpendicular half may experience a positive torque factor, being retarded in angular displacement from the lower half, while the latter at the same time crosses into a negative torque factor. The net result of the two divergent torque effects is a much lower net torque characteristic as shown in FIG. 4 curve C of my attached drawings.
My invention also provides means to combine tight shut-off with the other aforementioned advantages, in that its configuration (contrary to other "low torque" designs) allows an angular attachment, usually between 15.degree.-20.degree. to the vertical valve axis of the outer disc periphery to provide intimate contact and thereby greatly reduced valve leakage. The angle of contact is chosen depending on the type of material employed, that is, the tangent of the angle has to exceed the coefficient of static friction to avoid self-locking.
A further object of my invention is the ability to create a tight shut-off valve for so-called rubber lined butterfly valves. These valves consist usually of tubular steel or cast iron housings having either vulcanized or molded elastomeric inserts at their inner bores. It is customary to use cylindrical butterfly valve vanes for on-off purposes. Sealing is accomplished by squeezing an oversized vane into the rubber lined butterfly valve bore. This interference with, and displacement of, portions of the elastomeric insert provides the desired shut-off. However, one less desired by-product of this procedure is high static torque, also called "breakaway" torque. The other is wear of the liner material. While this breakaway torque is not considered too detrimental for hand operated valves, it nevertheless becomes highly objectionable for pneumatic operated modulating control valves. Any sudden forces changes, such as previously described, tend to cause dynamic instability and are very detrimental to the desired control function of such a valve.
My invention overcomes these difficulties by providing an angle seated vane wherein the elastomeric insert can be compressed with a relatively gentle force of the tilting vane periphery which avoids the squeezing action of a circular vane being pushed essentially in a direction parallel to the liner axis. As a result, the previously described breakaway torque is avoided and the operator torque required to provide shut-off cut in half.
Yet another objective of my invention is that the angular offset vane allows the hub portion to be sub-divided into two relatively short sections which eliminates the very expensive task of drilling a shaft bore through the complete diameter of the vane where the typical length of such bores is 12 times its diameter and requires the utilization of very special tools such as gun drills. My invention allows the use of conventional tooling since the bore distance can be kept to approximately two diameters.